To detect temporally, that is to say at what time takes place, the passage of a physical phenomenon and to measure the propagation velocity thereof, a measuring device conventionally used is for example an electrical sensor generally named “printed circuit”. Such a sensor comprises an electrical circuit which closes on the passage of a pressure wave. More specifically, such a sensor comprises copper tracks deposited on a substrate conventionally of Kapton®. On passage of a pressure wave, the track closes (which generates a short circuit) and thus delivers an electrical pulse of several tens of volts (via a pulse box) of which the temporal information is exploited. To be precise, knowing the position of each track, an average velocity of progression of the wave may then be computed. However, such a sensor is often sensitive to the amplitude of the pressure acting upon it and does not therefore always respond satisfactorily to weak action. It furthermore requires to be electrically supplied and may be sensitive to electromagnetic interference. Furthermore, the bulk of such a sensor is often such that it may present a constraint for implanting in a structure.
For approximately the last twenty years, devices for measuring physical phenomena increasingly often comprise an optical fiber sensor instead of an electrical sensor of the aforementioned type, which may be resistive or capacitive, for example to verify civil engineering infrastructures.
An optical fiber has the advantage of being insensitive to electromagnetic disturbance, of being relatively flexible and of low bulk (an optical fiber has a standard diameter of the order of approximately 250 μm at most). Furthermore, the sensor, an infrared light source and a detector may be situated several hundreds of meters away from each other since the optical fiber attenuates light around 1550 nm very little. To be precise, the attenuation is then of the order of approximately 0.2 dB/km.
Tests have for example been carried out with an optical fiber immersed in an explosive material (nitromethane). At the time of the explosion, a light beam injected into the fiber is reflected at a shock front which ablates the fiber. The measurement of the velocity or movement of the shock front is for example carried out using laser interferometry (for example using “LDI” velocity measurement, “LDI” standing for “Laser Doppler Interferometry”, or “PDV” for “Photonic Doppler Velocimetry”). The average velocity can be measured with this technique but the spatial locating of the event in the optical fiber remains difficult to determine. Furthermore, this technique operates best if the fiber is immersed in the explosive to have a concentric and symmetrical attack on the fiber. The implementation is thus difficult to manage, in particular in solid material.
It thus became apparent that the sensor is too rigid and is also fragile since the active part is bare.
Among optical fiber-based sensors, sensors using optical fiber with Bragg gratings have been developed for the most part in the last few years.
The operation of such a sensor relies on a measurement of an offset between a wavelength of a reflected beam of light relative to the Bragg wavelength of the grating. This offset varies according to the temperature or according to a stress applied to the fiber.
For the purpose of measuring disturbances linked to temperature, stresses (pressure variations) or deformations on materials or structures, devices comprising optical sensors with a Bragg grating have been developed. More recently, these sensors have been used to measure the passage of a detonation wave, or of a fast physical phenomenon, that is to say of which the propagation velocity is of the order of at least approximately one hundred meters per second. However, it is still difficult, to locate the position of the shock front precisely (that is to say for example with uncertainty less than approximately 1 mm) in the fiber at a given time with such devices.
Furthermore, such a sensor is still difficult to integrate within a material and/or a structure to be acted on and it is difficult to calibrate.
Furthermore, an optical fiber with Bragg gratings operates with difficulty when acted on asymmetrically.
It is also difficult to manufacture a grating with a length greater than one meter approximately, whereas the use of an optical fiber with Bragg gratings generally aims to have a grating with the largest possible length to continuously measure the change of a phenomenon. By way of information, currently, Bragg gratings generally measure a few centimeters. Such gratings, which are as long as possible, are furthermore generally chirped in order to make it possible to study change in reflection losses according to the wavelength progressively as the grating is destroyed.
Furthermore, to analyze the optical signals of these sensors, signal analysis methods are based on spectral measurements, either with a scanning technique (movement of a tunable laser coupled with a photodiode) or with an optical spectrum analyzer coupled to a broad spectrum source. However, the use of such techniques is limited, in particular considering their temporal resolution; they are for example inadequate for signal emissions at intervals less than approximately 1 μs.